OEmbed support for PreviewCard (#2337)

* OEmbed support for PreviewCard

* Improve ProviderDiscovery code failure treatment

* Do not crawl links if there is a content warning, since those
don't display a link card anyway

* Reset db schema

* Fresh migrate

* Fix rubocop style issues
Fix #1681 - return existing access token when applicable instead of creating new

* Fix test

* Extract http client to helper

* Improve oembed controller

app/helpers/http_helper.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module HttpHelper
+  USER_AGENT = "#{HTTP::Request::USER_AGENT} (Mastodon/#{Mastodon::VERSION}; +http://#{Rails.configuration.x.local_domain}/)"
+
+  def http_client(options = {})
+    timeout = { write: 10, connect: 10, read: 10 }.merge(options)
+
+    HTTP.headers(user_agent: USER_AGENT)
+        .timeout(:per_operation, timeout)
+        .follow
+  end
+end